1. Field of the Invention
The present invention relates to a high-voltage capacitor and a magnetron having a filter constituted of the high-voltage capacitor.
2. Discussion of Background
Well-known examples of high-voltage capacitors of this type in the prior art include those disclosed in Japanese Unexamined Patent Publication No. 1996-316099 and Japanese Unexamined Utility Model Publication No. 1992-40524. They have the following structural features in common. Two through holes are formed over a distance from each other at a dielectric ceramic material to constitute the capacitor. Individual electrodes that are independent of each other and a common electrode to be shared by the individual electrodes are provided at the two surfaces of the dielectric ceramic material at which the through holes open. The common electrode is fixed on a raised portion of a grounding metal by a means such as soldering. Through conductors are provided so as to pass through the through holes at the capacitor and through holes formed at the grounding metal. The through. conductors are soldered to the individual electrodes at the capacitor by using electrode connectors or the like. An insulating case is fitted around the external circumference of the raised portion of the grounding metal so as to enclose the capacitor. An insulating cover is fitted on the other side of the grounding metal so as to enclose the through conductors. The insulating cover is mounted so that it comes in close contact with the internal circumferential surface of the raised portion of the grounding metal. Then, a thermosetting insulating resin such as an epoxy resin is charged to fill the space inside the insulating case and outside the capacitor enclosed by the insulating case to assure satisfactory moisture resistance and insulation.
However, the insulating resin is simply charged in close contact with the internal and external circumferential surfaces of the raised portion of the grounding metal, the internal circumferential surface of the insulating case, and the internal and external surfaces of the capacitor, and the insulating resin is not bonded onto said surfaces. As a result, the stress which occurs while the insulating resin is becoming hardened and contracting, and the stress which repeatedly occurs during operation of the high-voltage capacitor, cause a gap to form between the insulating resin and the dielectric ceramic material and also induce formation of a gap between the insulating resin and the internal circumferential surface of the insulating case and between the insulating resin and the grounding metal.
Main causes of the above-mentioned gaps include the stress occurring while the thermosetting insulating resin such as an epoxy resin is becoming hardened and contracting and the stress attributable to the electrostrictive phenomenon occurring at the dielectric ceramic material constituting the capacitor.
The main constituent of the dielectric ceramic material constituting the capacitor is barium titanate. Such a dielectric ceramic material is a ferroelectric material that belongs to the piezoelectric crystal class. A ferroelectric material belonging to the piezoelectric crystal class imparts a reverse piezoelectric effect. As a result, when a high AC voltage is applied, mechanical energy is generated inside the dielectric ceramic material constituting the capacitor.
For instance, if this type of high-voltage capacitor is employed in a filter of a magnetron in a microwave oven, a. high AC voltage for oscillating the magnetron is applied to the capacitor. When such a high AC. voltage is applied to the dielectric ceramic material, the reverse piezoelectric effect mentioned earlier converts the electrical energy to mechanical energy. This results in the dielectric ceramic material expanding while the voltage is being applied and contracting to regain its original state when the voltage is not applied.
A voltage of approximately 4 kVO-P having a commercial frequency or a frequency in the range of 20 kHz to 40 kHz is applied to oscillate the magnetron. in a microwave oven. In addition, a transient voltage of 0 to 40 kVP-P is applied immediately before the magnetron starts to oscillate. In response to these AC voltages, the dielectric ceramic material constituting the capacitor repeats a process of expanding and contracting. This is referred to as an electrostrictive phenomenon of a dielectric ceramic material.
The electrostrictive phenomenon is the main cause of inducing formation of a gap between the dielectric ceramic material and the insulating resin and also inducing formation of a gap between the insulating resin and the internal circumferential surface of the insulating case and between the insulating resin and the grounding metal. Thus, high-voltage capacitors of this type in the prior art pose a risk of early defects occurring due to deterioration of characteristics and short circulation occurring between electrodes.
Another cause of deterioration of characteristics includes settling of a fire retardant into a groove formed in the dielectric ceramic material.
An epoxy resin containing hexabromobenzene that is an additional fire retardant has been used as a conventional insulating resin. In this case, the fire retardant comprises rod-like particles of large sizes, so the specific gravity of the fire retardant is greater than that of the base resin. As a result, the fire retardant is liable to settle onto the insulation interface to be unevenly distributed. The fire retardant itself is inferior to the base resin in insulating property and also adversely affects the bonding strength of the dielectric ceramic material and the base resin. In addition, the settling of the fire retardant causes problems such as variations in initial breakdown voltage and relatively early deterioration in high temperature load property.